The notion of providing VoIP services over a cellular wireless network has been proposed. However, supporting VoIP services over a conventional cellular wireless network, such as a third generation partnership project (3GPP) network, is challenging. A VoIP service is a variable low bit rate, delay and jitter sensitive application. The data rate for the VoIP services can vary from 8 kbps to 42 kbps depending on the codec rate and packet header compression.
A problem with VoIP over cellular networks is the large amount of overhead. Speech data for VoIP is likely to be carried by a real-time transport protocol (RTP). The VoIP packet, in addition to a link layer framing, includes an IP header (20 octets for IPv4), a UDP header (8 octets) and an RTP header (12 octets) for a total of 40 octets. With IPv6, the IP header is 40 octets for a total of 60 octets. The size of the payload depends on the speech coding and frame sizes being used and is somewhere between 15 bytes to 75 bytes.
In the 3GPP standards, packet switched domain, each packet contains control information, (both common and dedicated control information). The common control information includes scheduling information, user equipment (UE) identity and a transport format combination indicator (TFCI) of the packet (such as modulation and coding scheme (MCS) and packet size). The dedicated control information includes hybrid automatic repeat request (H-ARQ) process information and transmission sequence number. The control information adds significant overhead to the VoIP packet in the range of around 20%.
In long tem evolution (LTE) of the 3GPP standards, the physical layer air interface is orthogonal frequency division multiplexing (OFDM) multiple-input multiple-output (MIMO). Uplink and downlink resources comprise a set of sub-carriers. In the uplink, a varying modulation and coding scheme may support variable data rate that may not be sufficient to fit a packet within available resources. For example, a retransmitted packet may be successfully decoded if transmitted at a lower modulation and robust coding scheme. However, in this case, it may not be possible to fit the packet within the available resources.
Therefore, it would be desirable to provide a method for reducing the control overhead associated with a VoIP packet and for more flexibility to support a variable data rate.